I. Field of the Invention
The present invention relates generally to air supply systems using reciprocating type air compressors and, more particularly, to compressor unloading systems usable in such systems when they are used with and without an air dryer such as might be found in air supply systems for truck air brakes and the like. It is well known in the art that in such systems the air compressor operates in its pumping mode for only a short period of time, and that it is operated in what is known as an unloaded mode for approximately 70-90 percent of the time. How to unload the air compressor to save horsepower when the compressor is not pumping, without having other problems occur, has been a long standing problem in the art.
II. Description of the Prior Art
Air compressors used in medium and heavy duty vehicle air supply systems, such as those found in transport trucks, passenger buses, and the like, generally involve having an air compressor directly driven by the vehicle engine and supplied with inlet air at atmospheric pressure, or, in the case of many transport trucks today, turbocharged air from the engine turbocharger via the engine air intake manifold.
These air compressors generally are one, two or four cylinders in nature. The earliest attempts at internally unloading the air compressor in regard to two cylinder compressors was to simply hold the intake valves for both cylinders open and pump air back and forth between the cylinders, and this was relatively satisfactory when there was no concern with the fuel cost or the horsepower being lost.
With single cylinder air compressors, the earliest unloading method simply involved holding the intake valve open so that air was pumped in and out through the compressor intake opening.
In the European countries, the approach to the problem was somewhat different, and while the intake valve was left untouched, the compressor pumped all the time while the unloading function was performed external to the compressor via a separate diverted type system called an unloader valve. This method of unloading is now generally viewed as unsatisfactory because of the noise produced, and the horsepower losses incurred. Thus, design and development work focused on better ways to unload air compressors other than simply holding the intake valve open.
In many cases, when the intake valve was held open on a single cylinder and compressor and the compressor was supplied with naturally aspirated air, which was common at the time, since the compressor intake was supplied with a separate air cleaner, pumping back and forth through the air cleaner in many cases destroyed the air cleaner, or reduced its efficiency so it would not perform for the purpose for which it was intended.
This caused the truck manufacturers to switch to a system supplied with turbocharged inlet air from the engine turbocharger to avoid the air cleaner problems and assure a clean inlet air supply, but it was then found that when unloading in the conventional way, by holding the intake valve open, the faster the engine was running, the denser the air, and the more horsepower was lost due to the unloading. Thus, the search continued for a satisfactory solution to the problem.
A clutched compressor recently developed was not felt to be the answer to the horsepower loss problem because the compressor shaft normally runs through the compressor and often drives other accessories, such as the power steering, or engine fuel pump, and it was not satisfactory to have the power steering and other items nonfunctional for the time the compressor was unloaded. Also, the clutched compressor would substantially increase the cost of the air supply system, and this was not satisfactory. Thus, not much progress was made in developing a more efficient compressor unloading system, and until the present invention, the expedient of simply holding the intake valve open to allow unloading either to atmosphere, or the engine turbocharger, is the accepted way of unloading the air compressor.
This method of compressor unloading is known to be unsatisfactory when turbocharged inlet air is utilized for several reasons, including the loss of up to five (5) horsepower under certain operating conditions, the diversion of a significant amount of engine intake air through the compressor and air dryer out to the atmosphere when the compressor is in the unloading mode, and now with the advent of the air dryer in medium and heavy duty vehicle air brake systems, there is an inordinate loss of engine power, increased oil consumption, and waste of fuel in the unloading mode. Because of these new problems, past solutions were no longer acceptable by the engine and compressor makers, and much additional work and invention had to take place in the field of air compressor unloader systems before a satisfactory solution was arrived at. The inventions which have now been made, are disclosed and claimed in the present application, and in a copending application in which we are the inventors entitled RING VALVE TYPE AIR COMPRESSOR.